The invention relates to drilling fluids, drilling fluid additives and methods and, in particular, to drilling fluids, drilling fluid additives and methods useful for drilling oil wells through formations containing heavy oil or bitumen and useful for limiting or preventing tar sands accretion on metal surfaces.
Desirable properties of drilling fluids are well known and well documented in references e.g. “Composition and Properties or Drilling and Completion Fluids, 5th Edition” H. C. H. Darley et. al, Gulf Publishing Co. 1988. One of the desirable properties of a drilling fluid is to impart lubricity. Lubricity enables contact between the drill string and the earthen formation being bored or the drill string and steel well bore casings that have been inserted in the well bore without imparting undue or unmanageable torque increases due to frictional drag between the drill string and the surface it contacts. Commercially available lubricity additives are designed to provide lubricity between steel drill strings and consolidated earthen formations or steel drill string and steel well bore casings.
In horizontal drilling operations, the drill string first bores a vertical well, then at some predetermined point deviates from vertical in the build section. Eventually the borehole can deviate 90° from vertical to become a horizontal well. The horizontal section of a well is designed to have increase well bore intersection with the oil-bearing formation. The drill string in a horizontal section of the well experiences increased torque due to the increased contact between the string and the horizontal well bore. Horizontal wells are advantageous in heavy oil or bitumen production due to the low mobility of the oil. Horizontal wells offer inherent advantages over vertical wells in steam-assisted oil production. A significant portion of heavy oil deposits are found in unconsolidated formations, such as unconsolidated sand. The mixture of heavy oil or bitumen with this unconsolidated sand, often referred to as tar sands, makes the drill cuttings from these formations malleable. Deformation of these drill cuttings between the drill string and the formation or the drill string and the build section results in the adherence of a coating of the bitumen and sand on the drill string and casing. This coating is often referred to as accretion.
The accretion of the drill cuttings to the drill string and the casing impairs drilling operations by increasing the drag on the drill sting. The entire string can become stuck as a result of the increased drag resulting in excessive operational costs to drill the hole. Further, due to the unconsolidated nature of heavy oil/bitumen formation it is usually necessary to line the horizontal well bore by placing a slotted steel liner throughout the horizontal section. The slotted steel liner allows for oil production while at the same time limiting the amount of sand coproduced and maintaining the horizontal well bore integrity. The slotted steel liner is essentially a steel pipe with a typical wall thickness of 12 mm and a diameter close to that of the well bore. In order for the pipe to be placed in the horizontal section it must be pushed through the curved build section which will have a steel bore hole casing. If the intermediate casing is coated with the drill cutting accretions significant frictional resistance to landing the liner can be encountered to the point where some wells have had to be abandoned due to the slotted liner becoming immovable.
Treatments have been attempted in field operations to remedy these problems including additions of surfactants to emulsify the bitumen or prevent accretion, additions of inorganic salts to prevent accretion, additions of cleaning agents based on naturally occurring hydrocarbons such as d'limonene and additions of conventional lubricants. None of these treatments has satisfactorily prevented the aforementioned operational difficulties. Further, these additives are undesirable from an environmental point of view. Surfactants employed to emulsify the tar sand such as nonyl phenol ethoxylates are toxic to Microtox bacteria. In many instances the inclusion of hydrocarbons such as d'limonene is also undesirable and can present disposal problems with the aqueous based drilling mud. It can be seen then that an additive which would prevent accretion of the bitumen to metal surfaces in drilling these tar sands would be desirable and beneficial.